A Chilkat customer ran into this limitation while trying to send an MM7 message over HTTP. It can be solved by updating the AspMaxRequestEntityAllowed property in IIS 6.0:
AspMaxRequestEntityAllowed Metabase Property (IIS 6.0)
The AspMaxRequestEntityAllowed property specifies the maximum number of bytes allowed in the entity body of an ASP request. If a Content-Length header is present and specifies an amount of data greater than the value of AspMaxRequestEntityAllowed, IIS returns a 403 error response. This property is related in function to MaxRequestEntityAllowed, but is specific to ASP request. Whereas you might set the MaxRequestEntityAllowed property to 1 MB at the general World Wide Web Publishing Service (WWW Service) level, you may choose to set AspMaxRequestEntityAllowed to a lower value, if you know that your specific ASP applications handle a smaller amount of data.